This invention relates to circuitry for providing a DC output voltage, and, more particularly, to DC output voltage circuitry that has a substantially flat power supply rejection ratio across a frequency range of interest.
Electrical power in the form of voltage and current drives almost all modern day electronics. Two familiar sources of electrical power are the battery and the electrical power delivered to wall outlets. These are generally the only sources of electrical power that are ubiquitously available to consumers. Therefore, nearly all manufacturers of consumer electronics design their products to be powered by these sources.
Batteries and wall outlets provide electrical power in predetermined formats. Specifically, batteries provide a substantially constant voltage and uni-directional current. In contrast, wall outlets provide a periodic voltage and bi-directional current. However, electronic devices often require voltages and currents in different formats and levels than those available from a battery or wall outlet. An entire field of study, called power electronics, is devoted to the study of power and ways for providing voltage and current in different levels and formats.
One category of power electronic circuits is configured to produce a “DC voltage,” which is used herein to refer to a substantially constant voltage signal. The term “DC output voltage circuit” will be used herein to refer to a circuit that operates to provide a DC output voltage, when the operating conditions allow it to do so. In operation, a DC output voltage circuit may have some sensitivity to operating conditions (e.g., temperature), such that variations in the conditions may alter the value of the circuit's output voltage. Additionally, other circuits coupled to the DC output voltage circuit may introduce, for example, switching noise or frequency components into the DC output voltage circuit. Under consistent operating conditions and in the absence of frequency coupling, however, a DC output voltage circuit, as used herein, provides a substantially constant DC output voltage.
A DC output voltage circuit generally includes a power connection and an output connection. The DC output voltage circuit receives some voltage and current on the power connection and attempts to provide a DC voltage on the output connection. In some cases, a DC output voltage circuit can be designed in a way that mitigates or prevents voltage variations on the power connection from affecting the DC voltage on the output connection. One common way of quantifying this capability is by using a measure called the “power supply rejection ratio,” or PSRR, which is commonly measured in decibels. PSRR is a ratio of change in voltage on the power connection to a corresponding change in voltage on the output connection. A DC output voltage circuit that is more effective at mitigating the effects of the power connection's variations will have a higher PSRR. A DC output voltage circuit that is less effective at mitigating the effects of the power connection's variations will have a lower PSRR.
One existing method for designing a DC output voltage circuit includes using a negative feedback loop that compares the output voltage with a reference voltage. If the output voltage is greater than the reference voltage, the feedback loop operates to decrease the output voltage. If the output voltage is less than the reference voltage, the feedback loop operates to increase the output voltage. However, such designs almost always include an amplifier in the feedback loop, and it is known that amplifiers have poor PSRR for higher-frequency variations on the power connection. Therefore, DC output voltage circuits that use feedback may perform poorly in circumstances that involve high frequency noise on the power connection, for example. Another existing method for designing a DC output voltage circuit includes using what is known as a “diode-connected source follower,” which uses diodes connected to the gate of a transistor to set the DC output voltage. However, it is known that such circuits have poor PSRR for lower-frequency variations on the power connection. Therefore, DC output voltage circuits that use a diode-connected source follower may perform poorly in circumstances that involve low-frequency noise on the power connection, for example.
In the circuit implementations above, the deficiencies in the PSRR may cause variations in the DC output voltage in certain circumstances. Accordingly, these circuits may not be suitable for certain applications. At the same time, technology is increasingly progressing towards multipurpose devices and platforms that support diverse functionality, modes, and features. Most likely, these multipurpose platforms and devices will use different voltages and currents and will require the use of a DC output voltage circuit. However, for at least the reasons above, existing DC output voltage circuits may be incapable of accommodating at least some of these different applications. Accordingly, there is continued interest in improving the technology of DC output voltage circuitry at least from the point of view of greater applicability.